This invention relates to alarm systems for motor vehicles which prevent injury to nearby persons. More specifically, this invention relates to school bus alarm systems which warn a driver of the bus if a child is in danger of being injured when the bus starts moving.
Children are frequently injured at school bus stops, typically when boarding or deboarding the bus. Statistics maintained by the Fatal Accident Reporting System, for example, show that approximately 300 children are so injured--many fatally--in the United States per year. The injuries occur when a driver pulls away from the stop and is not aware that a child is in a zone near the bus that presents a high risk of injury. The zones beneath the bus as well as immediately in front of and behind the bus are blind spots for the driver of most buses and, therefore, the most dangerous areas.
Existing vehicle alarm systems typically transmit a signal in the danger zone and analyze the resulting received signal to detect doppler shifts which indicate a moving person near the motor vehicle. These systems, however, are difficult to use once the vehicle in which they are installed starts moving. For example, if a doppler effect system is monitoring the space beneath a slowly moving school bus whereinto a child has just moved, then the received signal will include a first doppler component resulting from ground reflections and a second doppler component resulting from the person's presence. These doppler components in the received signal should be separated for the alarm to be effective when the bus is moving. Doppler systems can also have limited coverage due to colocation of the transmit and receive antenna.